1. Field of the Invention
This invention pertains to AC corona chargers in general and in particular to AC corona chargers wherein an asymmetric voltage waveform is applied to the corona wires.
2. Description of the Prior Art
In an electrophotographic copying system, a photoconductive element is moved past a corona charger which applies a uniform, electrostatic charge to the photo conductive element. After leaving the vicinity of the corona charger, the photoconductive element moves past an exposure system at which it is exposed to a light image of an original, to cause the charge to be altered in an imagewise pattern to form a latent image charge pattern. Following exposure, the latent image charge pattern is developed by application of toner particles to the photoconductive element to create a toned image. Finally, this image is transferred from the photoconductive element to a receiver sheet and fused to form a permanent image.
AC charging typically uses a corona wire charger having a symmetrical AC voltage applied to the corona wires, superimposed on a DC offset voltage. A conventional AC charger operates at a 50% duty cycle, which is defined to mean that the time duration of the positive excursion of the AC component of the voltage waveform is equal to the time duration of the negative excursion. In general, duty cycle is defined as the percentage of time an AC component of the voltage waveform has a first polarity, compared to the time for one complete cycle. The AC component used for prior art charging is symmetrical and has essentially the same shape for both positive and negative excursions, e.g., sinusoidal, square, trapezoidal, or triangular waveforms. Typically, the maximum amplitudes of the positive and negative excursions of the AC voltage component are equal.
A grid is often used to control the surface potential of the photoconductor. The charging current is that current transmitted by the grid. It is well-known that grid-controlled AC corona chargers are considerably less efficient than grid-controlled DC corona chargers. The reason for this is that for a typical AC charger with grid control, the corona wire has the same polarity as the grid for only part of each cycle of the waveform. For an uncharged photoconductor element, charging current is only transmitted to the photoconductor in that portion of the AC waveform in which the emission current from the corona wire and the grid have the same polarity. Thus, charging is effectively in a pulsed DC mode. Charging continues in this mode until the surface potential of the photoconductor element approaches the potential of the grid. Typically, when the magnitude of the surface potential of the photoconductor is about 100 volts less than the grid potential, current of polarity opposite to that of the grid starts to be transmitted to the photoconductor element. As charging continues, the charging current contains an increasing proportion of current of opposite polarity, in an AC mode. When the photoconductor element is fully charged, the two components of current are equal.
Uniformity of charging is closely related to the uniformity of corona current emitted along the length of a corona wire. Charging uniformity is normally much higher with AC charging than with DC corona charging. For example, negative AC charging using a grid, at 50% duty cycle is significantly less noisy than negative DC charging. DC emitted currents typically show significant fluctuations at each position on a corona wire. These fluctuations are usually considerably worse with negative corona discharges than with positive corona discharges. Moreover, the sites of these fluctuations and their intensities may not be fixed spatially, but move around, or flicker, from place to place. Charging uniformity can be adversely affected by these fluctuations, resulting in unwanted density fluctuations or streaks in toned images, especially for negative charging. It would be desirable to have a corona charger with the efficiency of a DC charger and the uniformity of an AC charger.
U.S. Pat. No. 4,910,400 discloses a programmable DC charger with a high voltage corona wire between an electrode and a photoconductor. A voltage pulse is applied to the electrode, of the same polarity as the DC voltage applied to the corona wire, such that the corona charge produced by the wire is periodically accelerated by the electrode. The duty cycle of the pulsed voltage applied to the electrode controls the on-off time of the corona charger. U.S. Pat. No. 4,166,690 describes a power supply in which a digital regulator, in conjunction with at least one pulse width modulated power supply, permits fast rise times of the power supply current. This is useful in defining an interframe edge. U.S. Pat. No. 4,731,633 describes a corona charger, for positive charging, without a grid, in which a negative polarity voltage pulse is applied periodically to the corona wire for the prevention of positive streamer discharges, or "sheeting". This negative polarity voltage pulse is applied to the corona wire "in a manner having minimal effect on charging functions," for example, during the cycle-up period, cycle-out period, and standby period. An example is given in which a negative pulse duration of 20 ms follows a positive current signal pulse duration of 180 ms. This is equivalent to a positive duty cycle of 90%. This waveform has a frequency of 5 Hz, which is far outside of the usual range of AC operation and is used for operation between frames. U.S. Pat. No. 4,038,593 is for an AC power supply with regulated DC bias current. The duty cycle of the AC waveform is constrained, such that the time average of the voltage signal is essentially zero, i.e., the polarity of the voltage waveform which has a shorter duration has a higher amplitude. The regulation of the DC bias current is achieved without the use of a grid by varying the duty cycle. The DC bias current controls the level of charge on the photoconductor. U.S. Pat. No. 3,699,335 is for an apparatus that energizes a corona wire with voltage pulses of constant amplitude. The width or frequency of the pulses is controlled in response to an error signal to regulate the applied charge.